Compact optical systems that can observe a large area have been conventionally demanded as the objective optical systems for endoscopes. In order to realize such an optical system, it is necessary to reduce the entire length, the outer diameter, and the focal length. In particular, in an endoscope objective optical system used to observe thin lumen, such as the bronchi, the biliary tract, and the pancreatic duct, the most important thing is to reduce the diameter of the endoscope; therefore, optical systems for which a small diameter and short length are given priority over maintaining image quality and that consist of a few lenses have been adopted (for example, see PTLs 1 to 9).
As in PTLs 1 and 2, with a so-called retrofocus-type optical system in which a concave lens is disposed at the front end, a wider angle can be realized relatively easily. The above-described concave lens is formed into a meniscus shape or a plano-concave shape provided with a surface having a large refractive power on the image side and thus needs to ensure an increased thickness. Furthermore, in general, in the retrofocus-type optical system, the diameter of a front lens tends to increase.
In ultrasmall-diameter endoscopes, lenses whose outer diameters are 1 mm or smaller are often used. The lens processing becomes more difficult as the lens diameters are reduced, as well as the accurate processing is required for frames having a thickness of about 0.1 mm. Usually, lenses are held with a simply-structured frame, such as that shown in FIG. 13 or FIG. 15 of PTL 3, for example, and it is difficult to process a frame that has many step portions and that is formed by combining a plurality of members, such as that shown in FIG. 1 of PTL 4. From this view point, a configuration in which a concave lens is provided as the front lens is undesirable.
On the other hand, as a lens configuration for realizing a wide angle while being small in size, a configuration consisting of a stop, a front group having positive refractive power, and a rear group having positive refractive power is known (for example, see PTLs 5 and 6). This lens configuration has the advantage that the optical system can be reduced in length and diameter. However, as in PTL 6, for example, if lenses are disposed away from an imaging device, the number of places where air spaces are required increases, thus tending to increase the length of the optical system. Furthermore, in an oblique-incidence imaging device, because a convex lens is disposed at a position where the ray height is high, a sufficient distortion effect cannot be obtained, thus making it difficult to realize a wider angle. If one attempts to realize this by increasing the refractive power of the rear group, the curvature needs to be increased, thus deteriorating the ease-of-processing, and in that a sufficient edge thickness cannot be ensured, thus deteriorating the ease-of-handling. Therefore, in order to realize both ease-of-processing and a wide angle, it is preferable that the rear group having positive refractive power be disposed near the imaging device.
In addition, if the imaging device is a solid-state imaging device, a cover glass located on the surface thereof is not round in many cases; thus, a member serving as a guide for aligning the center of the imaging area with the center of the objective optical system is provided, and other members are mated with this member for alignment, thereby facilitating assembly (for example, see PTL 3). For this purpose, a flat glass or a plano convex lens attached directly on top of the imaging device is generally used. In contrast, in the case of the above-described lens configuration consisting of the stop, the front group having positive refractive power, and the rear group having positive refractive power, when the rear group having positive refractive power is made to serve as this guide, this is advantageous in terms of a reduction in size of the optical system.
From these circumstances, in order to realize both a reduction in size and a wider angle, as in PTLs 3 and 7, it is preferable to provide an optical system that adopts the lens configuration consisting of the stop, the front group having positive refractive power, and the rear group having positive refractive power and in which the lens group having positive refractive power in the rear group is joined to the imaging device.
In such an optical system, when strong light enters from outside the visual field, a ray striking an inner circumferential surface of a portion of a frame between air surfaces located between the front group and the rear group is reflected, thus tending to cause flare. For example, when performing ray tracing in a frame structure estimated from the drawings of PTL 3, it is confirmed that flare occurs, as shown in FIG. 24. In the figure, reference symbol GF denotes the front group, reference symbol GR denotes the rear group, reference numeral 1 denotes a frame for holding lenses in the front group, reference numeral 2 denotes a frame for holding a lens in the rear group, reference numeral 10 denotes a cover glass, and reference numeral 11 denotes a sealing glass provided on an imaging surface of the imaging device. Such flare occurs near the center of the visual field over a wide range.
Of course such flare can be prevented by increasing the lens diameter of the rear group to keep the inner circumferential surface of the frame far from the optical axis. However, the diameter of the optical system would be increased, the diameter of the lens in the rear group having positive refractive power, to be mated with the frame, would need to be increased, and the edge thickness of the lens would be reduced, thus deteriorating the ease-of-processing and ease-of-assembly.
Thus, in PTL 8, the rear group having positive refractive power is formed of a joined lens, and the joined lens is joined to an end surface of the imaging device, thereby inhibiting flare caused by light reflected at the inner circumferential surface of the frame. PTL 8 is based on the technological idea that a ring serving as the frame is omitted to accordingly increase the inner diameter of the frame, and a sufficient distance between the optical axis and the inner circumferential surface of the frame is not ensured; therefore, when a ray enters from outside the visual field, flare still occurs. This is clear from the fact that, in the ray diagram of FIG. 5 in PTL 8, a ray that does not reach the outermost off-axis position passes through a place close to the inner circumferential surface of the frame.
Furthermore, flare can be inhibited from occurring by changing the frame shape through light-blocking processing applied to the frame for a camera lens. However, from the above-described circumstances, it is difficult to apply such processing to a very small frame in an endoscope optical system, and accuracy is required, thus leading to a large increase in cost.
Similarly, a method in which a light-blocking member, such as a flare diaphragm, is disposed in the optical path can be considered. In order to realize this configuration by using a mask member, for example, because the optical system is very small, the light-blocking member itself has to be very small and very thin. Furthermore, it is necessary to provide the frame with a receiving portion to which the mask member is fixed, which inevitably makes frame processing more and more difficult, and a ray can strike the light-blocking member itself, causing even stronger flare depending on how surface treatment is done. In addition, when the mask member is fixed by bonding, because the bonding area is reduced due to a reduction in size, the fixing strength is very weak, thus making it easy for the mask member to peel off.
While there are limits to inhibiting flare by using a mechanical member, as described above, PTL 9 proposes a technology for reducing the occurrence of flare by providing the lens in the rear group with an inclined portion. PTL 9 is based on the premise that light strikes the inner circumferential surface of the frame; therefore, the amount of stray light that can be cut differs depending on the size of the inclined portion, and the intensity of flare changes significantly.
In particular, in the optical system for an ultrathin endoscope, because the lens itself for which the inclined portion is provided is small, small variations in the size of the inclined portion determine the intensity of flare, which makes it impossible to stably inhibit the occurrence of flare. Even when a sufficient size of the inclined portion is ensured in order to solve this problem, the inclined portion overlaps an area for necessary rays, thus reducing peripheral performance because of loss of light due to vignetting. Furthermore, the mating area where the lens in the rear group is mated with the frame is reduced, thus reducing the strength of the imaging unit.